Oneness, Uniqueness of Allah (Tawheed)

Bukhari :: Book 9 :: Volume 93 :: Hadith 566

Narrated Abu Huraira:

Allah's Apostle said, "For every Prophet there is one invocation which is definitely fulfilled by Allah, and I wish, if Allah will, to keep my that (special) invocation as to be the intercession for my followers on the Day of Resurrection."